It is important in many tactical missle gyro applications to know very accurately the alignment of the rotor spin axis of the gyro relative to some external mounting or reference surface. In some of these applications, the vehicle may be constrained to fly a course parallel to the gyro spin axis, so that an error in the alignment of the spin axis is a direct aiming error. It is usually desirable to keep such errors below one milliradian. Caging is the technique by means of which the rotor of the gyro is brought into a predetermined attitude or relation with respect to the housing or frame of the gyro.
In U.S. Pat. No. 3,416,378 there is disclosed a method of caging in which three individual pads are employed to form the caging plane. These three pads retract from the surface of the gyro rotor to be aligned independently, releasing it and permitting freedom of motion of the gyro rotor. Although this method of caging has proven successful, it is possible that because of the independent action of the three pads that one or more of the pads may have a slight delay in retracting thereby giving a misalignment impulse to the rotor surface, causing an alignment error. The use of three independent caging pads require the duplication of pistons, pins, pads and springs as well as gas passages communicating between the caging pads. It is readily seen that such a system requires a relatively high parts count and is excessively costly.